1. Field of the Invention
The present invention relates to an apparatus for correcting a camera shake.
2. Related Background Art
In a conventional camera, the angular velocity of a camera which is being inclined by an unintentional hand movement is detected, for example, by an angular velocity sensor. Blur in the object image caused by the camera shake is corrected by moving a correcting lens by a motor, and the like, at a speed in accordance with the angular velocity in a direction orthogonal with respect to the optical axis so that the movement of hands is canceled.
In such a camera the position of the correcting lens is detected, for example, by counting pulses output from interrupters. The motor for driving the correcting lens is controlled with a negative feedback according to the position of the correcting lens, thereby controlling the rotating speed of the motor, that is, the moving speed of the correcting lens.
The coefficient of static friction of the motor and machine parts for converting rotary motion of the motor into linear motion and transmitting it to the correcting lens is greater than the coefficient of dynamic friction thereof. Thus, higher voltage has to be applied to the motor in order to give greater force (greater acceleration) to the correcting lens when the correcting lens at rest is to be started moving than when it is already moving.
More specifically, suppose that turning on/off the motor (power applying time in a predetermined cycle) is controlled according to a PWM (Pulse Width Modulation) wave. When the motor and the correction lens at rest are to be started moving, the duty of the PWM wave is increased from 0%.
The duty of the PWM wave is defined as EQU (T/Tmax).times.100 (%)
wherein T is the current ON pulse duration of the PWM wave, and Tmax is the maximum ON pulse duration of the PWM wave.
In this case, however, even when the duty of the PWM wave is increased from 0%, the motor and the correcting lens do not start at once, because of the static friction of the motor and the machine parts. This is illustrated in FIG. 3, which shows the relation between the duty of the PWM wave and the moving speed of the correcting lens when rotation of the motor is in the steady state. As the duty of the PWM wave is increased from 0% (DO), the motor and the correcting lens at rest start to move at the time (D1) when the duty of the PWM wave reaches a certain value. Then, the moving speed of the correcting lens suddenly increases from 0 to a predetermined velocity V (D2).
After that, as the duty of the PWM wave is increased, the moving speed of the correcting lens is also increased. And when the duty of the PWM wave becomes 100%, the moving speed of the correcting lens reaches its maximum.
When the duty of the PWM wave is decreased from 100%, the moving speed of the correcting lens is also decreased. However, because the dynamic frictional force is smaller than the static frictional as described earlier, the correcting lens does not stop moving even when its moving speed is decreased to be equal to the velocity V at which the correcting lens starts moving. Only when the duty of the PWM wave is further decreased and the moving speed of the correcting lens comes to be equal to a velocity V' (D4) which is smaller than the velocity V, the correcting lens stops moving (D5).
As described above, when at first the duty of the PWM wave is increased from 0% to start the motor and the correcting lens moving, and then the duty of the PWM wave is decreased to stop the motor and the correction lens, the moving speed of the correcting lens changes in the following sequence: EQU D0.fwdarw.D1.fwdarw.D2.fwdarw.D3.fwdarw.D4.fwdarw.D5.fwdarw.D0
(see FIG. 3)
Accordingly, in the range from D4 to D7 (indicated by the dashed line in FIG. 3), that is, at a low speed, the movement of the correcting lens can not be controlled and the camera shake in taking pictures can not be corrected.
Further, in the range from D2 to D4, although the moving speed of the correcting lens can be controlled by varying the duty of the PWM wave if the motor and the correcting lens are moving, the moving speed of the correcting lens can not be controlled even by varying the duty of the PWM wave if the motor and the correcting lens are at rest, because in this case, the correcting lens won't be started moving.
In addition, in this kind of camera, whether the motor and the correcting lens are in motion or at rest is judged according to whether said interrupters have output a pulse within a last predetermined time lapse, for example several tens of milliseconds, or not. On the other hand, correction of the camera shake executed by controlling the moving speed of the correcting lens is a real-time operation.
Accordingly there is little time to control the moving speed of the correcting lens in the range from D2 to D4 if the correction is executed only after judging the motor and the correcting lens to be in motion.